dev-qt/qt3support does not build against musl. Same deal as in bug 551640, the fix is the same.
Created attachment 406194 [details, diff]
Add dev-qt/qt3support to the musl overlay
Committed to the musl overlay. Given your comment about qt4 support I don't suppose qt3 is still being supported. So I'm going to close this instead of marking it in progress while we push the patch upstream.